MAPPING THE MOTION OF AGGREGATES IN A THREE-PHASE AGITATED TANK CONTAINING A SUSPENSION OF NICKEL OXIDE IN ISO-OCTANE

Mapping the motion of aggregates in a three-phase agitated tank containing a suspension of nickel oxide in iso-octane
Gabriel Salierno1,2,3*, Mauricio Maestri1,2, Julia Picabea1,2, Miryan Cassanello1,2,
Cataldo De Blasio3, María Angélica Cardona4,5,6, Daniel Hojman4,5, Héctor Somacal6
1 Departamento de Industrias, Facultad de Ciencias Exactas y Naturales,
Universidad de Buenos Aires; Ciudad Universitaria - Buenos Aires, Argentina
2 CONICET-Universidad de Buenos Aires, Instituto de Tecnología de Alimentos y Procesos Químicos –ITAPROQ; Ciudad Universitaria - Buenos Aires, Argentina
3 Faculty of Science and Engineering, Åbo Akademi University - Vaasa, Finland
4 Laboratorio de Diagnósticos por Radiaciones, Dep. Física Experimental,
Comisión Nacional de Energía Atómica - San Martín, Buenos Aires, Argentina
5 CONICET - Buenos Aires, Argentina
6 Escuela de Ciencia y Tecnología, Universidad de San Martín,
San Martín, Buenos Aires, Argentina
*Email: Gabriel Salierno (gabriel.salierno@gmail.com)

Keywords: Radioactive Particle Tracking; Three-phase agitated tank; Turbulence

Industrial Application: Catalytic reactor engineering; Flue gas treatment

INTRODUCTION AND OBJECTIVES
The slurry agitated tank is widely used given its versatility and relative ease of installation. For instance, it is applied in catalytic reactions, fermentations, effluent treatment, and mineral processing (Kariyama et al., 2018; Prajapati et al., 2021). Due to its extremely complex fluid dynamics and the turbulent interaction of the different phases, stirred tank design is based on global correlations (Scargiali et al., 2014). This contribution explores the influence of agitation on the motion of solid aggregates that can be formed in the slurry. Features of the motion of a relatively big particle are directly determined from Radioactive Particle Tracking (RPT) measurements in a three-phase tank reactor containing iso-octane and nickel oxide powder with bubbling nitrogen.
MATERIALS AND METHODS
A double jacketed 3L glass reactor, stirred by a semi-axial Teflon vane, was used. The impeller is a pitched blade turbine with an inclination of 30° with respect to the vertical axis, commanded by an engine with a speed regulator. The agitation rate was set to 120, 150, and 200 rpm, and the impeller clearance is 0.05. Inside, the tank has three baffles to decrease the height of the vortices generated during agitation. The reactor contains 2.3L of iso-octane (ρ = 690 kg/m3; μ = 0.5 mPa.s) and nickel oxide powder (ρ = 6670 kg/m3; dp = 80 μm) at a bubbling speed of nitrogen fixed at 10 mL/min. The effluent is bubbled in a liquid N2 cold finger to avoid sending iso-octane to the environment.
RPT experiments are carried out with an array of 8 NaI(Tl) 2" scintillation detectors, distributed around the vessel as two squared aligned strata of four detectors. Figure 1 (left) shows the experimental equipment in operation during RPT measurements. Due to poor wettability, the nickel oxide powder agglomerates to sizes ranging up to 2 mm. Consequently, the RPT tracer must represent this aggregate, which will have a significantly larger particle size and a lower density than the density of the powder particles. In this context, the most suitable RPT tracer is a grain of 24NaCl (ρ = 2160 kg/m3; Figure 1 - right), covered with a polymer layer to keep its integrity and for safety purposes.

Figure 1: Experimental equipment (left) and 24NaCl radioactive tracer for RPT (right).
RESULTS AND CONCLUSIONS
The time series of instantaneous velocities can be calculated by differentiating successive positions from the tracer trajectories, dividing by the sampling period. Mean velocity fields can be determined as the average of the tracer velocities when visiting a certain parcel of space. In the same way, the turbulence kinetic energy (TKE) distribution can be determined from the correlation matrix of the ensemble average of velocities (Salierno et al., 2018). Figure 2 allows comparing the distribution of normalized aggregates hold up () and TKE in the frontal plane, centered on the axis of symmetry of the column, for the same solids content condition.

Figure 2: Sagittal view of the solid hold up (o: 1.3 g/L) and TKE at different stirring conditions.
RPT enables studying the three-dimensional motion of agglomerates and the distribution of stresses. A clear increase in the overall intensity of the turbulence and the space occupied by tracer trajectory is observed with increasing stirring speed.
REFERENCES

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